(i) Field of the Invention
This invention relates to heat withstanding cooling elements for lining of metallurgical furnaces above the hot metal and slag levels, in particular the steel scrap and/or sponge iron melting Electric Arc and Plasma Arc Furnaces.
(ii) Description of the Prior Art
Flexibility of operation, high productivity rates per unit and overall better production economy of the melting Electric Arc Furnaces for iron and steelmaking, has resulted in world wide rapid growth of the number and size of this type of furnace. Recently Electric Arc Furnaces have gained even more importance, when becoming the main units for large scale steel production by further enlarging their capacities and particularly by increasing the power input levels. More frequently the increased power input levels are achieved not only by higher electrical power loading, but also with the help of additional energy sources such as oxy-fuel burners and most recently Plasma Arc burners. As a result of the increased power input levels the life of standard interior wall refractory lining above the hot metal zone has become extremely short. As a consequence of the short life of interior wall refractory, limited to approximately 100-150 heats, furnace operation has been frequently interrupted for the removing of the worn out, and installation of the new, lining. The time delay necessary for replacement of the refractory lining had an obvious negative effect on the overall furnace operating productivity and costs. Development and practical use of water cooled elements for interior walls as a replacement for standard refractory walls, above the hot metal zone, has reduced the non-productive time delays and has significantly improved the operating economy of the Electric Arc Furnace. Several design types of furnace interior wall water cooled elements have already been made available to the steel industry.
Some of the known designs of the box type water cooled elements (U.S. Pat. Nos. 3,940,552 and 4,119,792) are made from plain metallic plates, with added components which protrude from the elements hot face for better slag retention. A disadvantage of the box type water cooled elements is that, due to the large cross-section of the cooling water passages, the desirable high exposure of the hot face plate of the element to the cooling water flow is reduced. Together with low cooling water flow velocity, inadequately directed water flow creates vortexes and dead flow spaces in the passage corners with a consequent water vapor and air-bubble development and concentration, causing frequent and significant localized heat transfer reduction. This in effect could lead to destruction of the water cooled element, resulting in water leakage and necessitating replacement of the element. Another disadvantage of the box type elements, made from plain metallic plates is that the applied slag retaining system, consisting of multitudes of specially shaped components of separate design and protruding from the hot face of the element, are vulnerable to damage and burn-off during normal operation of the furnace. These components must be often renovated, since the hot face plain plate of the box type element has insufficient slag retaining ability. An additional disadvantage of the box type water cooled element made from plain metal plate of restrictd thickness is the low structural strength inherent in the design, which does not allow high pressure cooling water operation, necessary for possible heat recuperation from the used cooling water.
Other known designs of box type water cooled elements (U.S. Pat. Nos. 4,097,679 and 4,122,295) are made from combination of plain metallic plates and cast or heavy wall hot face plate in which for better slag retaining the cast hot face plate has precast grooves or ribs, and in which the heavy wall plate has grooves or ribs created by machining. Besides having disadvantages similar to the box type elements made from plain metallic plates only, the combination box type water cooled elements are more vulnerable to cracking of the heavy wall hot face plate. Furthermore the grooves or ribs created by precasting or machining and protruding from the hot face of the element are exposed to extremely high thermal stresses in the area of their roots at the transition into the body of the hot face plate.
Due to the rapid operational temperature changes of the furnace interior, the protruding components break off from the hot face plate. In addition to the fact that the hot face plate of the element becomes plain and therefore unable to efficiently retain a protective slag layer, the area of the broken-off ribs is destroyed resulting in further cracks of the hot face plate and a variety of consequential damages.
Water cooled elements constructed as solid cast block-slabs, with internally arranged water cooling pipes (U.S. Pat. No. 3,843,106) have the disadvantage, that their substantial wall thickness and weight, necessitates strengthening of the furnace shell resulting in overall weight increase. Inadequate heat dissipation from the smooth hot face of the element, because of the wall thickness, further enhanced by rapid changes of furnace operational temperature, prevents the desired build-up of a protective slag layer. This leads to destruction of the monolithic cast body made from cast iron, copper or other metal. Exposed water cooling pipes with low thermal inertia eventually completely separate from the main cast block body, enforcing premature replacement of the element. An additional disadvantage of cast in pipes is the gap between the pipes and cast block, which results from the casting procedure and creates a thermal barrier reducing even more the heat extracting ability of the element.
The known design of tubular, coil type water cooled elements with neighbouring tubes of the coil arranged in a contacting relation with tube center lines in one plane (U.S. Pat. No. 4,207,060) have a significant disadvantage in that they have insufficient ability to permanently retain protective refractory material applied or deposited prior to operational use, or slag deposited on the element hot face by splashing during furnace operation.
This disadvantage exists even in the case where the smooth surface of the hot face of the tubes is provided with a plurality of projections or burls. The lack of satisfactory permanent retention of the initially, deposited refractory or splashed on slag on the hot face in this type of water cooled element rests, in principle, in the physical phenomenon of high shrinkage gradient of the slag layer body, which is almost directly proportional to its internal thermal gradients.
In practical application, the parallel neighbouring tubes, arranged in a closely adjacent-contacting relation form between them a prismatic space of a generally triangular cross-section. Since the intensive cooling effect of the tubes is practically uniform around their perimeters, the mass of the triangular prism of the slag deposited between the tubes is exposed to a highly non-uniform cooling, causing high internal thermal gradients resulting in excessive internal stresses. Amplified by rapid changes of furnace interior temperature, the protective refractory or slag layer cracks and spalls, exposing the unprotected hot face of the tubes of the coil to seaparate or combined mechanical, electrical and thermal damage to the hot face of the tubes of the element.
Another known design of tubular water cooled element consists of plurality of, in one plane to each other parallel tubes arranged with a space between neighbouring tubes and interconnected with flat bar spacers on the cold face of the cooling element (German Offenlegungsschrift DE 29 37 038 A 1). This type of water cooled element also has the disadvantage of being unable to permanently and satisfactorily retain the initially deposited protective refractory or splashed on slag layer. Cracking and spalling of the protective refractory or solidified slag layer is initiated in principle by excessive thermal expansion of the interconnecting flat bar spacers causing separating forces between the deposited refractory or slag and the parallel tubes arranged in one plane.
All previously discussed known designs of water cooled elements have a significant common disadvantage, in addition to those described:
flow of liquid coolant such as water is constant through all phase-periods of the furnace operating cycle, although thermal loading of elements hot face varies from 0 to maximum.